It is in my opinion best to pick s formation and keep to it, build your team around it but choose a formation that is versatile that can change to another will very little alteration!, I use 3-1-4-2 because I can change to 4-4-2, 4-3-3 or 4-1-3-2 , 3-1-3-1-2 depending how the game is going, for this to work best you need a full 22 man team with players that have multiple positions.